All of a sudden, my laptop asks for a password - what do I do?

Are you logging in with a Microsoft Account (Email address) or a Local Account, where you made up the user name and password?
If a Microsoft Account, go to https://accounts.microsoft.com. Try to login here, if that fails, click Forgot Password button, you will have a choice to either answer your security questions or they will send you a code to another Email address or Smart Phone.
If it is a Local Account, we cannot help you with Passwords as that is against forum rules.
Did you try leaving the Password field Blank?

I suggest you post a photo showing the dialog asking for a password.

You created a user account when you installed Windows; it is part of the process.
How have you previously been logging in to your user account in Windows?
- Logging in to your user account is something that happens every time you start Windows. It is possible to automate it so perhaps you had previously done so.
- If your user account has a blank password then this allows automated login so perhaps that is what has previously happened.


Denis

A password is basic and fundamental to your account. Even if you have set up your PC for passwordless login (skip the login prompt, automatically log in as a certain user - see relevant tutorial) there are circumstances in normal usage when you will be asked for a password.

E.g. offline System Restore or command prompt
(That's where you use advanced startup options, or boot your PC from a Win 10 install disk to advanced startup options).

I cannot tell you why you were suddenly asked for one when you weren't previously- my crystal ball is currently down for maintenance for the foreseeable future.

Given that it sounds as if your PC may be an older one (model not stated), consider running basic disk and file system checks for starters.

Should you need it, there's a tutorial to help you reset your (forgotten) password.

You created a user account when you installed Windows; it is part of the process.

"I did? I do not recall that."

Yes.

If you left its password blank then when Windows starts it will, in effect, reach the login screen, read what's in the password box [nothing], decide that [nothing] is the correct password and log you in without bothering you at all.

- - - - - - - -

I cannot know for certain why the intermittent password problem has occurred but it does indicate that there might be a problem in your user profile [the file that Windows reads & that contains all the data about your user account]. Intermittent login problems are a common precursor to permanent login failures.

I urge you to create one or two spare, local, password-protected Admin accounts now so that you are prepared for any eventual failure. You will be able to use one of them to login if your existing user accounts ever gives up the ghost completely.
See my ditty - Create two spare local, password-protected Admin accounts [post #2] - TenForums
- And do remember to write their passwords down somewhere secure yet accessible.
- You cannot omit passwords for additional accounts even though Windows would let you. They are your last defence against online hackers trying to sneak onto your computer behind your back.

Just in case you are not convinced, this is what I would have to advise you to do if you had a permanent login failure without having made those spare accounts beforehand.
Fix UAC prompt has greyed out or missing Yes button - TenForumsTutorials
[The name of the tutorial merely reflects the first symptom that many users report]

All the best,
Denis